The present invention relates to a method for measuring the total concentration of carbon monoxide and hydrocarbons in oxygen by ion mobility spectrometry.
Oxygen is widely employed as a reacting gas in the integrated circuit industry, in order to build up oxide layers generally acting as an electric insulation between different active portions of a circuit. As is known, in the manufacture of these devices, the purity of all the used materials has a basic importance. As a matter of fact, contaminants possibly present in the reactants or in the reaction environment may be incorporated into the solid state devices, thus altering their electrical features and giving rise to production wastes. The purity specification of the gases employed in the manufacturing process may change among different manufacturers and depending on the specific process the gas is employed in. Generally, a gas is considered to be acceptable for manufacturing purposes when its impurities content does not exceed 10 ppb (parts per billion, namely an impurity molecule per 109 total gas molecules). Preferably, the impurities content is lower than 1 ppb. It thus becomes important to have the possibility to measure extremely low concentrations of impurities in the gases in an accurate and reproducible way.
A technique that can be exploited for such purpose is ion mobility spectrometry, known in the art under the abbreviation IMS. The same abbreviation is also used for the instrument the technique is performed with, indicating in this case xe2x80x9cIon Mobility Spectrometerxe2x80x9d. The interest in such a technique comes from its extremely high sensitivity, associated with limited size and cost of the instrument. By operating under suitable conditions, it is possible to detect species in a gas medium, in the gas or vapor phase, in amounts of the order of picograms (pg, namely 10xe2x88x9212 g) or in concentrations of the order of parts per trillion (ppt, equivalent to one molecule of analyzed substance per 1012 molecules of sample gas). IMS instruments and analytical methods in which they are employed are disclosed, for instance, in U.S. Pat. Nos. 5,457,316 and 5,955,886, assigned to the U.S. firm PCP, Inc.
The physicochemical basis of the technique is very complicated, just as the interpretation of the IMS analytical results. For an explanation of this basis and results, reference can be made to the book G. A. Eiceman and Z. Karpas, Ion Mobility Spectrometry, CRC Press (1994).
Briefly, an IMS instrument essentially consists of a reaction zone, a separation zone and a charged particle collector.
Within the reaction zone there occurs the ionization of the sample, comprising gases or vapors to be analyzed in a carrier gas, usually by xcex2-radiation emitted by 63Ni. The ionization mainly occurs on the carrier gas, with the formation of so-called xe2x80x9creactant ions,xe2x80x9d whose charge is then distributed to the species present depending on their electron or proton affinities or on their ionization potentials.
The reaction zone is divided from the separation zone by a grid which, when maintained at a suitable potential, prevents the ions produced in the reaction zone from entering into the separation zone. The analysis xe2x80x9ctime zeroxe2x80x9d is established by the moment when the grid potential is annulled, thus allowing the ions admission into the separation zone.
The separation zone comprises a series of electrodes, which create such an electric field that the ions are carried from the grid toward the collector. In this zone, maintained at atmospheric pressure, a gas stream is present having an opposite flow direction with respect to the direction of the ion movement. The counter-flow gas (defined in the field as xe2x80x9cdrift gasxe2x80x9d) is an extremely pure gas, which may either correspond to the gas whose impurities content is to be determined, or may be a different gas. The motion velocity of the ions depends on the electric field and on the cross-section of the same ions in the gaseous medium, so that different ions take different times for crossing the separation zone and reaching the particle collector. The time elapsed from xe2x80x9ctime zeroxe2x80x9d to the time of arrival on the particle collector is called xe2x80x9ctime of flight.xe2x80x9d The collector is connected to the signal processing system, which transforms the current values sensed as a function of time into the final graph, where peaks corresponding to the different ions are shown as a function of the xe2x80x9ctime of flight.xe2x80x9d From the determination of this time and the knowledge of the test conditions, it is possible to trace the presence of the substances which are the object of the analysis, whereas from the peak areas it is possible to calculate, through suitable computation algorithms, the concentration of the corresponding species.
In the most common mode, an IMS analysis is carried out on species having a positive charge. In the case of oxygen, in contrast, oxygen forms negative species in the reaction zone. Under such conditions in the IMS analysis (negative mode) only species can be sensed having electron affinity higher than oxygen and then being able to receive a charge from this gas. This essentially occurs in the case of carbon dioxide (CO2). The analysis of impurities in oxygen is therefore limited. Among the species whose concentration in oxygen is interesting to measure, there are for instance carbon monoxide (CO) and hydrocarbons, particularly methane (CH4).
An object of the present invention is to provide a method of measuring the total concentration of CO and hydrocarbons in oxygen by ion mobility spectrometry.
According to a first embodiment of the present invention, this object is reached through a method comprising the following steps:
a) converting carbon monoxide and hydrocarbons, present in the oxygen stream, into carbon dioxide;
b) measuring the concentration of carbon dioxide in the oxygen after the conversion according to step a); and
c) deducing from the measurement of step b) the total initial concentration of carbon monoxide and hydrocarbons.
According to a second embodiment of the invention, the method is employed in the case of oxygen initially already containing carbon dioxide as an impurity (such condition can be ascertained through a preliminary indicative test performed on oxygen without previously submitting the same to the conversion step of CO and hydrocarbons). In this case, a concentration value will be obtained in the IMS analysis corresponding to the sum of the originally present CO2 and that coming from the conversion of CO and hydrocarbons. In this case, a variation of the method of the invention is employed comprising the following steps:
a) converting carbon monoxide and hydrocarbons, present in the oxygen stream, into carbon dioxide;
b) measuring the concentration of carbon dioxide in the oxygen stream after the conversion according to step a);
bxe2x80x2) performing a further measurement of carbon dioxide concentration in the oxygen stream not submitted to the conversion step according to step a); and
c) deducing from the comparison of the carbon dioxide concentrations measured in steps b) and bxe2x80x2) the initial concentration of carbon monoxide and hydrocarbons.